The invention disclosed herein pertains to enhancing the flow of coolant air through the plenum of an avionic tray and a line replaceable unit that is mounted to the tray.
In aircraft, electronic and electrical circuitry is contained in line replaceable units (LRUs), which are essentially metal boxes having air inlet and air outlet holes on their bottom and top surfaces, respectively. The LRUs are inserted in trays whose dimensions and configurations are standardized in accordance with the requirements of the avionic and aerospace industries. A multiple circuit electrical connector is typically mounted in an upstanding wall in the rear part of the tray, and mating electrical connectors are mounted on the rear of the LRU box, so that when the LRU is slid into the tray, the mating connectors join and a plurality of electrical circuits are completed. Generally, an assortment of cables run out of the back of the connectors to other locations in the aircraft. The bottom, or base, of the tray has a plenum chamber beneath it and a recess whose margins are surrounded by a gasket. There is a so-called metering plate in the bottom of the recess. The metering plate has a plurality of holes, some of which may be plugged, so that the quantity of air flowing outwardly from the plenum is suitable for the size of the LRU box and for the heat from the electronic components in the LRU, which must be dissipated by way of coolant air flowing through the LRU. The LRU bottom surface has a plurality of air inlet holes and its top surface has a plurality of air outlet holes. Provision is made for mounting a fan on the tray. The fan draws air from the atmosphere within the aircraft and discharges the air into the plenum. After passing through the holes in the metering plate, the air enters the LRU through its bottom holes, and, after absorbing heat from the electronic components within the LRU box, the air discharges through the holes in the top of the LRU box. Factors which must be considered in connection with designing the coolant system for LRUs are the space on the tray that is available for mounting the fan, fan size, fan speed and fan efficiency. Together, these factors must be chosen so that the system will provide the LRU with adequate coolant air to carry away the substantial amount of heat that is produced by the electronic components in the LRU. Maximizing the plenum fan diameter or size is likely to be governed by the space available for the fan. Heretofore, fulfillment of the coolant air quantity requirement was still only marginal in many cases even though the largest permissible diameter fan having the highest speed and highest efficiency was selected. In some cases fans having extremely high rotational speeds emitted a shrill, annoying noise. Moreover, as fan speeds go up, fan motor life shortens. These, and other considerations, raise a challenge as to what can be done to meet air flow requirements when the largest, highest speed and most efficient fan that is available is insufficient or leaves an unacceptably low margin of thermal safety.
Another consideration is that there are usually several stiff electrical cables running from the electrical connector on the rear wall of the avionic tray. These cables necessarily run through space that is shared with the coolant air fan and its mounting. Hence, the size and configuration of the fan mounting or, in other words, the size and configuration of the adapter that makes the transition between the fan and the plenum of the tray ought to be optimized.